The present invention is concerned with a novel process for the manufacture of symmetrical, terminally ring-substituted polyenes, especially carotenoids of the canthaxanthin, astaxanthin, etc., types and the corresponding 2,2xe2x80x2-dinor-carotenoids (generally referred to as 4,4xe2x80x2-diketo-carotenoids) from acetalized polyene dialdehydes by an acid-catalyzed condensation reaction with cyclic dienol ethers.
Lewis acid-catalyzed additions of xcex1,xcex2-unsaturated ethers (aldehyde enol ethers) to acetals are known and date back to the work of Muller-Cunradi and Pieroh (see U.S. Pat. No. 2,165,962). Hoaglin and Hirsch [J.A.C.S. 71, 3468 et seq. (1949)] investigated this reaction further and broadened the possible applications, which Isler et al. likewise did in the nineteen fifties with respect to the synthesis of xcex2-carotene, crocetin dialdehyde, lycopene, as well as, xcex2-apocarotenoids [Helv. Chim. Acta 39, 249 et seq. and 463 et seq. (1956), ibid. 42, 854 et seq. (1959) and U.S. Pat. Nos. 2,827,481 and 2,827,482]. Later, Mukaiyama [Angew. Chem. 89, 858 et seq. (1977) and Org. Reactions 28, 203 et seq. (1982)] extended the reaction by using the readily accessible trimethylsilyl enol ethers.
Also enol ethers of aliphatic and alicyclic ketones, including alkyl enol ethers and silyl enol ethers, react with acetals to give xcex2-alkoxy-ketones or, with cleavage of alcohol, to give the corresponding elimination products [Chem. Lett. 1974, 16 et seq., J.A.C.S. 102, 3248 et seq. (1980), Chem. Lett. 1987, 1051 et seq. as well as ibid., 1975, 569 et seq.].
The first Lewis acid-catalyzed condensations of 1-alkoxy-1,3-dienes (dienol ethers) with xcex1,xcex2-unsaturated acetals were reported by Nazarov and Krasnaya [J. Gen. Chem. USSR 28, 2477 et seq. (1958)] and by Makin [Pure and Appl. Chem. 47, 173 et seq. (1976), J. Gen. Chem. USSR 31, 3096 et seq. (1961) and 32, 3112 et seq. (1962)]. Here, the coupling of the acetal to the dienol ether takes place as far as can be seen exclusively at its xcex3-position with the formation of a chain-lengthened xcex1,xcex2-unsaturated acetal, which, however, in competition with the first acetal reacts with further dienol ether with the formation of a Pa/27.7.1999 further, chain-lengthened (xcex1,xcex2-unsaturated acetal, etc. [telomer formation; see also Chemla et al., Bull. Soc. Chim. Fr. 130, 200 et seq. (1993)]. For this reason such a condensation has been found not to be workable for synthetic purposes, especially for the synthesis of apocarotenals [Isler et al., Adv. Org. Chem. 4, 115 et seq. (1963)].
1-Alkoxy-1,3-dienes and trimethylsilyloxydienes [of the CH2xe2x95x90CHxe2x80x94CHxe2x95x90CHxe2x80x94OSi(CH3)3-type] can be condensed with xcex1,xcex2-unsaturated acetals in the presence of Lewis acid catalysts, as disclosed by Mukaiyama et al. in Chem. Lett. 1975, 319 et seq. In this coupling, the attack also seems to take place exclusively at the terminal (xcex3) carbon atom of the diene system [xe2x80x9cxcex3-attackxe2x80x9d; Mukaiyama et al., Bull. Chem. Soc. Japan 50, 1161 et seq. (1977) and Japanese Patent Publication (Kokai) 36,645/1977/Chem. Abs. 87, 201825 t, (1977)]. In contrast to the reaction with 1-alkoxy-1,3-dienes, in which an xcex1,xcex2-unsaturated acetal results, the reaction of trimethylsilyloxydienes with acetals forms an aldehyde that does not react further with the diene (no telomer formation). Thereby, zinc bromide and many other Lewis acids are required as catalysts only in small amounts [Fleming (et al.), Tetr. Lett. 1979, 3209 et seq. and Chimia 34, 265 et seq. (1980) as well as Brownbridge, Synth. 1983, 85 et seq]. By using this method, Mukaiyama et al. were able to synthesize vitamin A [Kokai 36,645/1977, Chem. Lett. 1975, 1201 et seq. and Bull. Chem. Soc. Japan 51, 2077 et seq. (1978)] and workers from Rhxc3x4ne-Poulenc developed new routes to carotenoids and vitamin A [German Patent Publication, i.e., Deutsche Offenlegungsschrift (DOS), 2,701,489 and A.E.C. Socixc3xa9txc3xa9de Chimie Organique et Biologique No. 7824350].
The aforementioned Lewis acid-catalyzed condensation of a dienol ether with an xcex1,xcex2-unsaturated acetal based on the work of Nazarov and Krasnaya, Makin, and Chemla et al. would be a very valuable access to apocarotenals and bis-apocarotenals if the yield of the desired primary product of the . . . CHxe2x95x90CHxe2x80x94CH(Oalkyl1)xe2x80x94CH2xe2x80x94CHxe2x95x90CHxe2x80x94CH(Oalkyl1)(Oalkyl2)-type could be increased and the telomer formation could be suppressed. Thus, the desired polyene aldehyde of the . . . CHxe2x95x90CHxe2x80x94CHxe2x95x90CHxe2x80x94CHxe2x95x90CHxe2x80x94CHO-type could be obtained from this primary product by hydrolysis of the acetal group C(Oalkyl1)(Oalkyl2) and elimination of alkyl1OH [European Patent Publication (EP) 0 816 334 A1].
Some examples are known wherein ketone dienol ethers of the . . . CHxe2x95x90CHxe2x80x94CHxe2x95x90C(O alkyl/trimethylsilyl)-CH2-alkyl-type are reacted with aldehydes, acetals, orthoesters and other electrophiles to give xcex1,xcex2-unsaturated ketones of the . . . Exe2x80x94CH2xe2x80x94Hxe2x95x90CHxe2x80x94COxe2x80x94CH2-alkyl-type (E represents an electrophilic substrate) [Tetr. Lett. 22, 705 et seq. and 2833 et seq. (1981), ibid., 27, 2703 et seq. (1986), ibid. 29, 685 et seq. (1988) as well as Chem. Ber. 123, 1571 et seq. (1990)]. The usefulness of this reaction appears to be somewhat limited, not on reactivity grounds, but because of the difficult accessibility of the aforementioned ketone dienol ethers, because, inter alia, regioselectivity problems have to be taken into consideration in their production [formation of the undesired regioisomers of the . . . CH2xe2x80x94CHxe2x95x90CHxe2x80x94C(O-alkyl/trimethylsilyl)xe2x95x90CH-alkyl-type].
Based on the aforementioned dienol ether condensation, A. Rxc3xcttimann has recently developed a novel, economical synthesis of apocarotenals and bis-apocarotenals (EP 0 816 334 A1) that is advantageous because the Cxe2x80x94C linkage is effected under catalytic conditions, namely using a Lewis acid catalyst. Moreover, no phosphorus- or sulphur-containing reagents are required in this approach.
A novel synthesis of canthaxanthin, astaxanthin, the corresponding 2,2xe2x80x2-dinor-carotenoids and structurally similar, symmetrical carotenoids having two terminal rings (4,4xe2x80x2-diketo-carotenoids) has now been found. This novel synthesis is likewise based on a catalyzed dienol ether condensation and also avoids the use of phosphorus- and sulphur-containing reagents, but makes use in a very refined and surprising manner of a cyclic compound as a reaction participant that has not only the main features of the terminal ring, but also the dienol ether grouping required for the condensation.
An object of the present invention is to manufacture the aforementioned symmetrical carotenoids starting from polyene diacetals while avoiding, as much as possible, the aforementioned disadvantages of the state of the art and replacing the Wittig, Horner or Julia reaction hitherto used for this purpose. This object is achieved in accordance with the invention by reacting a polyene diacetal with a cyclic dienol ether in the presence of a suitable catalyst, namely a Lewis acid or Brxc3x6nsted acid, and, after hydrolyzing the resulting condensation product, undertaking a base- or acid-induced elimination of alcohol at the two ends of the mainly conjugated hydrocarbon chain bonded to the two rings in order to obtain the desired symmetrical, terminally ring-substituted fully conjugated polyene. Not only is the reaction of the cyclic dienol ether with the polyene diacetal novel, but, surprisingly, it is effected with an exclusive attack of the acetal at the xcex3-position of the dienol ether. By the base- or acid-induced elimination of the alkanol subsequent to the hydrolysis, two conjugated Cxe2x80x94C double bonds are formed without the need for a phosphorus- or sulphur-containing reagent, which is in contrast to the methodology hitherto usually employed in this field.
Accordingly, the present invention is concerned with a process for the manufacture of a symmetrical, terminally ring-substituted polyene of formula I 
wherein
R1 is hydrogen or hydroxy;
m is 0 or 1; and
n is 0, 1, or 2
which comprises reacting a polyene di(O,O-dialkyl acetal) of formula II 
wherein
R2 is C1-6-alkyl and
n is as defined above,
with a cyclic dienol ether of formula III 
wherein
R3 is hydrogen;
R4 is C1-4-alkoxy, or
R3 and R4 together form an optionally substituted methylenedioxy group, xe2x80x94Oxe2x80x94C(R5)(R6)xe2x80x94Oxe2x80x94, wherein R5 and R6 are each independently hydrogen, C1-4-alkyl or phenyl, and
m is defined as above
in the presence of a Lewis or Brxc3x6nsted acid, and hydrolyzing the reaction product under acidic conditions and cleaving off the alkanol R2OH from the thus-obtained compound of formula IV 
wherein
R1 is hydrogen or hydroxy, depending on whether R3and R4in formula III each are hydrogen or C1-4-alkoxy or together signify the optionally substituted methylenedioxy group, and
R2, m and n have the significances given above,
under basic or acidic conditions.
Additionally, when a cyclic dienol ether of formula III, wherein R3 is hydrogen and R4 is C1-4-alkoxy, is used, a compound represented by formula V is an intermediate product of this first process step: 
wherein
R2 is C1-6-alkyl;
R4 is C1-4-alkoxy;
m is 0 or 1; and
n is 0, 1, or 2.
Alternatively, using a cyclic dienol ether of formula III wherein R3 and R4 together form the optionally substituted methylenedioxy group, the intermediate product of the first process step is a compound of the general formula 
wherein
R2 is C1-6-alkyl;
R5 and R6 are each independently hydrogen, C1-4-alkyl, or phenyl;
m is 0 or 1; and
n is 0, 1, or 2.
As used throughout the scope of the present invention, the term alkyl as used in, e.g., xe2x80x9cC1-4-alkyl,xe2x80x9d xe2x80x9cC1-6-alkyl,xe2x80x9d and xe2x80x9cC1-4 alkoxyxe2x80x9d embraces straight-chain and branched groups such as, methyl, ethyl, isobutyl, and hexyl. The term xe2x80x9clowerxe2x80x9d as used in, e.g., xe2x80x9clower aliphatic ether,xe2x80x9d embraces C1-20, preferably C1-10.
In the case of the cyclic dienol ethers of formula III there comes into consideration a substituted cyclopentene (m is 0, so that formula III then represents specifically the formula 
or a substituted cyclohexene (m is 1, so that formula III in this case specifically represents the formula 
In this sense, there are to be understood under the corresponding terminal cyclic groups (rings), which the compounds of formulae I and IV have, groups of the formulae 
wherein * denotes the respective linkage position.
The formulae of polyenes and cyclic dienol ethers disclosed in the scope of the present invention embrace isomeric forms, e.g., optically active and cis/trans or E/Z isomers, as well as mixtures thereof unless expressly stated to the contrary. With respect to the E/Z isomerism, the (all-E) isomers of the polyene di(O,O-dialkyl acetals) of formula II and of the products of formula I of the process in accordance with the invention are, in general, preferred.
In the first process step of the process in accordance with the present invention, i.e., the reaction of the polyene di(O,O-dialkyl acetal) with the cyclic dienol ether under acidic conditions, an exclusive attack of the former compound at the xcex3-position of the cyclic dienol ether takes place. When a cyclic dienol ether of formula III, wherein R3 is hydrogen and R4 is C1-4-alkoxy, is used, an intermediate product of this first process step is a compound of the general formula 
Alternatively, using a cyclic dienol ether of formula III wherein R3 and R4 together form the optionally substituted methylenedioxy group, the intermediate product of the first process step is a compound of the general formula 
This first process step is conveniently carried out by reacting the polyene di(O,O-dialkyl acetal) of formula II with the cyclic dienol ether of formula III in an organic solvent at temperatures in the range of about xe2x88x9250xc2x0 C. to about +60xc2x0 C. (e.g., from about xe2x88x9225xc2x0 C. to about +60xc2x0 C.), preferably in the temperature range of about xe2x88x9230xc2x0 C. to room temperature (e.g. from about 0xc2x0 C. to room temperature), and in the presence of a Lewis or Brxc3x6nsted acid. Suitable organic solvents are, in general, polar or non-polar aprotic solvents. Such solvents are, for example, lower halogenated aliphatic hydrocarbons, e.g., methylene chloride and chloroform; lower aliphatic and cyclic ethers, e.g., diethyl ether, tert.butyl methyl ether and tetrahydrofuran; lower aliphatic nitrites, e.g., acetonitrile; lower aliphatic esters, e.g., ethyl acetate; as well as, aromatic hydrocarbons, e.g., toluene. The preferred solvent is acetonitrile, optionally in combination with further aforementioned solvents, especially with ethyl acetate or methylene chloride. Where a mixture of acetonitrile with ethyl acetate or methylene chloride is used, the ratio by volume of acetonitrile to ethyl acetate or methylene chloride is preferably about 1:1 to about 4:1, particularly about 4:1. Examples of Lewis acids that can be used are zinc chloride, zinc chloride dietherate, zinc bromide, zinc di(trifluoromethanesulphonate), titanium tetrachloride, tin tetrachloride, boron trifluoride etherate as well as iron(III) chloride; and examples of Brxc3x6nsted acids that can be used are p-toluenesulphonic acid, methanesulphonic acid, trifluoromethane-sulphonic acid, sulphuric acid, as well as, trifluoroacetic acid. In general, the Lewis acids, especially the zinc salts, boron trifluoride etherate, and iron(III) chloride, are preferred. The catalysts are, in general, used in catalytic (below stoichiomeric) amounts, conveniently in an amount which is about 0.5 to about 30 mol percent based on the amount of polyene di(O,O-dialkyl acetal) used, with the mol percent range preferably lying between about 5% and 10%. Further, there are conveniently used about 2.1 to about 4 equivalents, preferably about 2.2 to about 2.6 equivalents, of cyclic dienol ether per equivalent of polyene di(O,O-dialkyl acetal). Moreover, the reaction is conveniently effected at normal pressure with, in general, the pressure not being critical.
Frequently, the intermediates of formula V or of formula VI occur, usually together with diverse similar intermediates, as a precipitate, which can be isolated after cooling the reaction mixture, for example to about xe2x88x9210xc2x0 C. to xe2x88x9220xc2x0 C., and filtration. Subsequently, the intermediate is then hydrolyzed with aqueous acid to the corresponding compound of formula IV.
When no isolation and subsequent hydrolysis is undertaken, a direct hydrolysis in the reaction mixture can be carried out. In so doing, an acid, preferably slightly dilute aqueous acetic acid, for example, with a ratio by volume acetic acid:water of about 9:1, is added to the reaction mixture and the mixture is subsequently stirred for a period, for example, about 30 minutes to about 2 hours, in the temperature range of about 0xc2x0 C. to about 50xc2x0 C. In addition to acetic acid, p-toluenesulphonic acid can also be used in a catalytic amount, such as about 1-2 mol percent based on the amount of polyene di(O,O-dialkyl acetal) used, in order to accelerate the hydrolysis. In comparison to the separate hydrolysis of the intermediate product of formula V or VI, the hydrolysis undertaken directly in the reaction mixture is preferred.
The product of formula of IV can be isolated from the reaction mixture and, if desired, purified in any manner per se. Typically, the mixture is combined with water and the whole is extracted with a water-immiscible organic solvent, such as, for example, a lower alkane, dialkyl ether, or aliphatic ester, e.g., hexane, tert.butyl methyl ether, or ethyl acetate, and the organic phase is washed with water and/or sodium bicarbonate solution and/or saturated aqueous sodium chloride solution, dried and concentrated. The thus-isolated and, at least to some extent, washed crude product can then, if desired, be purified further, for example, by column chromatography, e.g., using eluting agents, such as, hexane, ethyl acetate, toluene, or mixtures thereof, or recrystallization, for example from an alcohol, e.g., methanol or ethanol.
With respect to the last process step, i.e., the cleavage of the alkanol R2OH from the compound of formula IV, eliminations of the alkanol from xcex2-alkoxyaldehydes or xcex4-alkoxy-xcex1,xcex2-unsaturated aldehydes with the formation of the corresponding xcex1,xcex2-unsaturated aldehydes are known in the specialist literature and can be carried out under a variety of conditions. For example, in the field of known base-induced eliminations, 1,8-diazabicyclo[5.4.0]undec-7-ene is very often used as the base in an amount of about 2 to 4 equivalents based on the amount of aldehyde used. Such conditions are used in the known production of carotenoids [see, inter alia, Bull. Chem. Soc. Japan 50, 1161 et seq. (1977), ibid. 51, 2077 et seq. (1978), Chem. Lett. 1975, 1201 et seq. and DOS 2,701,489] and of vitamin A (see, inter alia, Chem. Lett. 1975, 1201 et seq). As examples of acid-induced alkanol cleavages reference is again made to Bull. Chem. Soc. Japan 50, 1161 et seq. (1977) and to J. Gen. Chem. USSR 30, 3875 et seq. (1960) in which p-toluene-sulphonic acid or 85% phosphoric acid is used as the acid catalyst. The buffer system sodium acetate/acetic acid [Helv. Chem. Acta. 39, 249 et seq. and 463 et seq. (1956) and U.S. Pat. Nos. 2,827,481 and 2,827,482] has been used for such a cleavage especially in the production of the carotenoids. Also, in the case of corresponding alkoxy ketones (xcex2-alkoxyketones or xcex4-alkoxy-xcex1,xcex2-unsaturated ketones), the cleavage of the alkanol in general succeeds very well: see in this respect Synthesis 1986, 1004 et seq. or J. Org. Chem. 49, 3604 et seq. (1984). Taking into consideration this and other pertinent literature a person skilled in the art will have no difficulty in finding reaction conditions for the successful performance of the last step of the process in accordance with the invention.
Furthermore, the cleavage of the alkanol R2OH (2 equivalents per equivalent of the compound of formula IV) can also be carried out with several equivalent amounts of a base based on one equivalent of the compound of formula IV. Thus, the last process step in this case is conveniently carried out by converting the compound of formula IV, dissolved in a suitable organic solvent, into the corresponding polyene of formula I in the presence of a base with cleavage of the alkanol R2OH. Suitable organic solvents are, in general, protic or aprotic solvents or mixtures thereof, such as, for example, alcohols, e.g., ethanol and isopropanol, and alcohol mixtures; or aromatic hydrocarbons, e.g., toluene. The base can be inorganic or organic, and, in general, there are suitable strong bases, especially those alkali metal alcoholates that are stronger bases, e.g., sodium ethylate. As indicated above, there are conveniently used at least two equivalents of base per equivalent of the compound of formula IV, preferably about 2.5 to about 8 equivalents.
When an alkali metal alcoholate is used as the base, either a solution of the sodium alkoxide in the alkanol is prepared in advance or this solution is prepared freshly from metallic sodium and the alkanol. The bringing together of the alkanolic solution of the sodium alkoxide with the solution or suspension of the compound of formula IV in the (same) alkanol, preferably likewise prepared in advance, can be effected in either sequence, as desired. The reaction mixture is then stirred while heating, suitable in the temperature range of about 60xc2x0 C. to about 140xc2x0 C., preferably at temperatures of about 80xc2x0 C. to about 100xc2x0 C. Depending on the boiling point of the solvent, the reaction is conveniently effected at normal pressure or with a slight excess pressure (in order to achieve the desired temperature), with, in general, the pressure not being critical. Under these conditions the cleavage reaction has normally finished after a few hours, especially after about 5 to 10 hours.
In the case of an acid-induced alkanol cleavage, suitable acids are, in general, strong mineral acids, such as, for example, hydrochloric acid, hydrobromic acid, hydroiodic acid, sulphuric acid, and perchloric acid, and sulphonic acids, such as, for example, methane-sulphonic acid, trifluoromethanesulphonic acid, and p-toluenesulphonic acid. The mineral acids can be aqueous and, depending on the acid, can have a concentration of about 10 to about 50%. Hydrochloric acid (especially about 10 to 37%), hydrobromic acid (especially about 25 to 30%) or hydroiodic acid (e.g., 47%) are the most suitable. In this case only a catalytic amount, i.e., up to a maximum of 1 equivalent per equivalent of the compound of formula IV, preferably about 0.1 to about 1 equivalent, is required. Further, the acid-induced alkanol cleavage is effected in a solvent in which the compound of formula IV has a good solubility (a so-called xe2x80x9chomogeneous cleavagexe2x80x9d) or in a solvent in which this is not the case, i.e., in which the compound of formula IV is on the other hand in suspension (xe2x80x9cheterogeneous cleavagexe2x80x9d). In both cases, however, the acid catalyst need not be completely dissolved. Suitable solvents for the homogenous cleavage are especially halogenated aliphatic hydrocarbons, e.g., methylene chloride, chloroform and 1,2-dichloroethane, and aromatic hydrocarbons, e.g., benzene and toluene. Suitable solvents (dispersion media) for the heterogeneous cleavage are lower aliphatic nitrites, ketones, and carboxylic acids, e.g., acetonitrile, acetone, and acetic acid, preferably acetonitrile and acetone. In both cases, the alkanol cleavage is conveniently effected in the temperature range of about xe2x88x9220xc2x0 C. to about +50xc2x0 C., preferably in the range of about 0xc2x0 C. to room temperature. The reaction time is, in each case, dependent on the reaction temperature and can be several hours, with the cleavage reaction normally having finished at the latest after about 5 hours.
Such an acid-induced alkanol cleavage is more suitable than a base-induced alkanol cleavage for the manufacture of astaxanthin, i.e., the compound of formula I, wherein R1 is hydroxy and m and n are both 1.
As an alternative to the separate hydrolysis and alkanol cleavage, these two process steps can be carried out in a combined process step without isolation of the compound of formula IV using a somewhat stronger acid, especially a mineral acid, such as, for example, aqueous hydrochloric acid.
Irrespective of the chosen procedure for the last process step, the product can be isolated from the reaction mixture in any manner known per se, normally by cooling the reaction mixture, conveniently to room temperature or even to about 0xc2x0 C., optional addition of water and filtration. After its isolation the product can be washed, for example with water and/or aqueous alcohol, and finally, if desired, dried under reduced pressure. If desired, further methods such as column chromatography and recrystallization can be used in order to arrive at a still purer product. Where an isomerization of the Z-isomers present in the product to the corresponding E-isomers is desired, a respective intermediate step can be included in the isolation and purification process. This intermediate step comprises adding an alcohol or aqueous alcohol, e.g., aqueous isopropanol, immediately after the cooling, heating the mixture in the temperature range of about 80xc2x0 C. to about 100xc2x0 C. and cooling the mixture and filtering off and drying the solid. Saturated lower hydrocarbons, e.g., heptane, also come into consideration as possible solvents. In general, the E-isomers are less soluble than the corresponding Z-isomers and accordingly often occur as a precipitate in higher yield. Furthermore, as mentioned above, the (all-E)-isomers of the products of formula I are, in general, preferred.
In the process in accordance with the invention defined above R2 is preferably methyl, R3 is preferably hydrogen and R4 is preferably isobutoxy, or R3 and R4 together are preferably the methylenedioxy group (R5=R6=hydrogen), and n is preferably 1.
While some of the educts of the process in accordance with the invention are known, others can be produced according to methods known per se from precursors that are, to some extent, known.
For example, the novel polyene di(O,O-dialkyl acetals) of formula II can be prepared very simply in a known general manner by reacting the corresponding polyene dialdehyde of the formula 
with the respective trialkyl orthoformate, especially in the corresponding C1-6-alkanol, e.g., methanol for the O,O-dimethyl acetal, and in the presence of a catalytic amount of an organic acid or of a Lewis acid, e.g., p-toluenesulphonic acid, or zinc chloride [see, for example, Organikum, Organisch-chemisches Grundpraktikum, 6th edition, p. 377 et seq. (1963)]. The reaction takes place to some extent in suspension, i.e., the respective polyene dialdehyde is suspended in the alkanol or in an alkanol/methylene chloride mixture and then there are added to the suspension about four mol equivalents of the trialkyl orthoformate, followed by a trace of acid catalyst, e.g., p-toluenesulphonic acid. In so doing, the dialdehyde dissolves slowly and the polyene di(O,O-dialkyl acetal) of formula II, which is formed simultaneously, crystallizes out slowly. The reaction is conveniently carried out in the temperature range of about 0xc2x0 C. to about 40xc2x0 C. and, as a rule, takes from 30 minutes to about 4 hours. As further background that illustrates the generally known acetalization method, reference is made to European Patent Publications 252 389 and 391 033, as well as, to J. Mol. Cat. 79, 117 et seq. (1993).
The polyene dialdehydes of formula VII, in turn, are either known, especially from the specialist literature concerning carotenoids, or, where novel, can be produced according to methods known per se. Thus, for example, the two-fold reaction of 2,7-dimethyl-2,4,6-octatriene-1,8-dial (the so-called xe2x80x9cC10-dialxe2x80x9d) with C5- or C10-Wittig aldehydes to give different chain-lengthened dialdehydes has become known from this literature. The text books xe2x80x9cCarotinoidsxe2x80x9d (O. Isler, published by Birkhxc3xa4user Basel and Stuttgart, 1971, especially chapters VI and XII and the further literature mentioned therein) and xe2x80x9cCarotinoids, Volume 2: Synthesisxe2x80x9d (G. Britton, S. Liaaen-Jensen and H. Pfander, published by Birkhxc3xa4user Basel Boston Berlin, 1996, especially chapters III and VII), yield much useful information on the production and the occurrence of the known dialdehydes.
The cyclic dienol ethers of formula III are novel and represent a further aspect of the present invention.
Those cyclic dienol ethers of formula III, wherein R3 is hydrogen, R4 is C1-4-alkoxy and m is 0, can be prepared in accordance with Reaction Scheme 1, starting from the known 2-methyl-1,3-cyclopentanedione, as follows: 
Those compounds of formula X wherein R4 is methoxy or isobutoxy are known and can be produced according to the method of Rosenberger et al. [J. Org. Chem. 47, 2134 et seq. (1982)] from the commercially available 2-methyl-1,3-cyclopentanedione/1-hydroxy-2-methyl-cyclopenten-3-one of formula VIII by acid-catalyzed etherification with methanol or isobutanol to the corresponding compound of formula IX, followed by a double methylation with methyl iodide and lithium diisopropylamine at low temperature, e.g., at about xe2x88x9270xc2x0 C. p-Toluenesulphonic acid/toluene is especially suitable as the acid catalyst/solvent combination for the first process step, VIIIxe2x86x92IX, and tetrahydrofuran is preferably used as the solvent for the second process step. The remaining compounds of formula X wherein R4 is C1-4-alkoxy other than methoxy or isobutoxy can be produced in an analogous manner.
In accordance with the Peterson olefination [J. Org. Chem. 33, 780 et seq. (1968)], the keto enol ether of formula X is then reacted with trimethylsilylmethyllithium (itself produced from trimethylsilylmethyl chloride and metallic lithium in pentane) in pentane, and, after subsequent addition of water, gives the compound of formula XI in crystalline form. Subsequently, this can be converted directly with potassium hydride as the base and in tetrahydrofuran as the solvent at temperatures below room temperature, e.g., in the range of about 0xc2x0 C. to about 15xc2x0 C., into the desired cyclic dienol ether of formula IIIa. In so doing, the pentane used as the solvent in process step Xxe2x86x92XI is replaced distillatively by the solvent of the last process step XIxe2x86x92IIIa (tetrahydrofuran) until a boiling point of about 62xc2x0 C. (boiling point of tetrahydrofuran 66xc2x0 C.) is attained. It is not necessary to isolate the compound of formula XI produced as the intermediate: by the solvent exchange and the thermal treatment this compound decomposes into the desired cyclic dienol ether of formula IIIa and the lithium salt of trimethylsilanol.
After the addition of water, the thus-obtained dienol ether is conveniently extracted with a suitable solvent, especially a lower alkane, e.g., pentane or hexane, or a lower aliphatic ether, e.g., diethyl ether, and thereafter purified by distillation under a high vacuum.
Those cyclic dienol ethers of formula III wherein R3 is hydrogen, R4 is C1-4-alkoxy and m is 1 can be produced in accordance with Reaction Scheme 2 as follows: 
Those compounds of formula XIII wherein R4 is methoxy, ethoxy, or isobutoxy are known [Tetr. Lett. 37) 1015 et seq. (1996) and, respectively, EP 31875] and can be produced according to the method of Rosenberger et al. [J. Org. Chem. p47, 2130 (1982)] from methyl isobutylate and ethyl vinyl ketone (by a Robinson annulation) followed by an acid-catalyzed etherification of the resulting 1-hydroxy-cyclohexen-3-one of formula XII with the corresponding alkanol to give the corresponding compound of formula XIII. Methanesulphonic acid or p-toluenesulphonic acid are especially suitable as the acid catalyst for the last process step XIIxe2x86x92XIII and a lower alkane, e.g., hexane, or an aromatic hydrocarbon e.g., benzene or toluene, is especially suitable as the solvent. The remaining compounds of formula XIII wherein R4 is C1-4-alkoxy other than methoxy, ethoxy, or isobutoxy can be produced in an analogous manner.
The third and the last process step to the desired cyclic dienol ether of formula IIIb can be carried out analogously to process step Xxe2x86x92XI and process step XIxe2x86x92IIIa for the production of the corresponding 5-ring compound [in accordance with the Peterson olefination, J. Org. Chem. 33, 780 et seq (1980)]. Although the compound of formula XIV can be isolated and purified by crystallization, it is, however, very unstable, especially in pure crystalline form; it readily rearranges in air into the compound of the formula 
Therefore, the compound of formula of XIV after crystallization and drying, conveniently under a high vacuum and while gassing with inert gas, e.g., argon, must be used as soon as possible in the next (last) process step. This last process step is conveniently effected in the presence of potassium hydride as the base and in tetrahydrofuran as the solvent at temperatures in the range of about 0xc2x0 C. to about 15xc2x0 C.
After the addition of water, the thus-obtained dienol ether is conveniently extracted with a suitable solvent, especially a lower alkane, e.g., pentane or hexane, or a lower aliphatic ether, e.g., diethyl ether, and purified thereafter by distillation under a high vacuum.
This production of the cyclic dienol ether of formula IIb is carried out more efficiently by reacting the compound of formula XIII with trimethylsilylmethyllithium in pentane at temperatures of about 0xc2x0 C. to about xe2x88x9210xc2x0 C. and, thereafter, replacing the pentane by tetrahydrofuran until the boiling point of tetrahydrofuran is achieved; this is thus a one-pot process. As in the above-described production of the cyclic enol ether of formula IIIa, the resulting intermediate of formula XIV decomposes into the desired cyclic enol ether of formula IIIb and the lithium salt of trimethylsilanol. Heating in tetrahydrofuran for too long a time should, however, be avoided, since under the prevailing basic conditions a partial isomerization of the resulting cyclic enol ether into the corresponding cyclohexadiene of the formula 
can take place.
Finally, those cyclic dienol ethers of formula III wherein R3 and R4 together form an optionally substituted methylenedioxy group, xe2x80x94Oxe2x80x94C(R5)(R6)xe2x80x94Oxe2x80x94, can be produced in accordance with Reaction Scheme 3, starting from the known 1,5-dihydroxy-2,4,4-trimethyl-cyclopent-1-en-3-one or 1,6-dihydroxy-2,4,4-trimethyl-cyclohex-1-en-3-one, as follows: 
The compound of formula XVII is acetalized in a manner known per se with a ketone R5COR6 or its dimethyl acetal to give the corresponding compound of formula XVIII [see Helv. Chim. Acta 64, 2436 et seq. (1981) and EP 0 085 158 A2]. Where acetone or its dimethyl acetal is used as the ketone or dimethyl acetal, the thus-obtained compound of formula XVIII wherein R5 and R6 are both methyl and m is 0 or 1 is known. However, formaldehyde or formaldehyde dimethyl acetal is preferably used as the acetalizing reagent, thereby producing the compound of formula XVIII, wherein R5 and R6 are both hydrogen, in high yield. The next two process steps XVIIIxe2x86x92XIX and XIXxe2x86x92IIIc can be carried out analogously to process steps Xxe2x86x92XI and XIxe2x86x92IIIa or XIIIxe2x86x92XIV and XIVxe2x86x92IIIb of Reaction Schemes 1 or 2. The preferred product of this process is 4,6,6-trimethyl-5,6,7,7a-tetrahydro-5-methylidene-1,3-benzodioxol, i.e., the compound of formula IIIc wherein R5 and R6 are both hydrogen and m is 1.
Novel compounds within the scope of the present invention include the cyclic dienol ethers of formula III and the intermediates of formulae IV, V, and VI.
The final products of the process in accordance with the invention, i.e., the symmetrical, terminal ring-substituted polyenes of general formula I, belong, for the most part, to the carotenoid field and can be used in a corresponding manner, for example as colorants or pigments for foodstuffs, egg yolk, the integuments (especially skin, legs and beak) and/or the subcutaneous fat of poultry, the flesh and/or the integuments (especially skin, scales and shell) of fish and crustaceans, etc. For example, astaxanthin is predominantly suitable as a pigment for the pigmentation of salmon. This use can be effected according to methods known per se as described, for example, in European Patent Publication No. 630,578.